Diabetes mellitus in rats is associated with enhanced intestinal absorption of glucose, but the kinetic basis for this observation is disputed. Only recently has it been appreciated that failure to account for the resistance of the unstirred water layer (UWL) leads to gross errors in the estimation of the kinetics of active and passive transport. Accordingly, the uptake Jd (unidirectional flux rate) of glucose into the intestine was measured in alloxan- or streptozotocin-diabetic rats (DM) and in control animals (C) under conditions selected to yield low and high UWL resistance. When the UWL resistance was minimized by stirring the bulk phase, Jd was higher in DM than in C, but only from the higher concentrations of glucose; this difference between DM and C was obscured when UWL was high. The difference between C and DM in Jd from 40 mM glucose progressively increased from 3 to 83 days after induction of DM, and was present in the jejunum and ileum as well as in young and old animals. The Jd from 0.5 to 2 mM glucose was similar in DM and C, and the inhibitory effect of galactose on Jd of glucose was similar in DM and C. Jd was higher in DM than in C for L-glucose and for D-glucose in the presence of phlorizin and in the absence of sodium ion. Kinetic analysis demonstrated a higher maximal transport rate (Jdm) and apparent passive permeability coefficient (P*) in DM than in C, but the apparent affinity constant was unchanged. It is concluded that (1) the greater Jd of glucose in DM is due to a greater Jdm and P*, rather than due to changes in the properties of the glucose carrier itself, and (2) these differences may be obscured by variations in UWL

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